If you are the supervisor of this employee, you will have to take them into your office and explain why the job was posted. If they are being fired, but you want to keep them until someone new comes on, that doesn't seem very sporting. Give them the chance to find a new job as soon as possible. You're not playing fair.
It will depend with the questions that you will be asked. You need to read and understand more about the job and also its responsibilities or what you are expected to do so as to be able to answer any questions if approached.
I agree with Koichan. Explain to them why you thought handling it that way was REALLY the best way to handle things and I hope you see it wasn't . Also, you may have some serious legal problems now with the situation and how you handled it. You need to handle it by telling this employee you will offer them all the help they need in keeping their job for right now and follow the rest of Koichan's advice . Why did you do that? Why did you handle it like this?